1. Field of the Invention
The present invention relates to ink jet printers, and, more particularly, to a paper position sensing system for an ink jet printer.
2. Description of the Related Art
With printers which use a columnar array of print elements or nozzles, a typical mode of operation requires that the column of nozzles be swept horizontally across the paper while the nozzles are selectively fired according to points in a bitmap which represent printed pixels. At the end of such a swath, the paper is indexed vertically by the height of the printhead and the printhead is again swiped across the paper. With this process, there are inherent print defects introduced by such things as paper feed inaccuracies and nozzle-to-nozzle variations in drop size or placement.
Studies of inkjet printer print quality indicate that the paper positioning system must be able to control the location of the paper within 4 micrometers to eliminate paper positioning artifacts in single pass printing. A control system capable of this resolution requires a position sensor with resolution in the sub-micrometer range. Conventional digital encoders, the usual sensor for this type of control system, are not capable of achieving resolution this fine.
An optical encoder 10 (FIG. 1) includes a light source 22, a code wheel 24, a light detector 26, an optical mask 28 and a rotating shaft 30. Mask 28 discerns the spatial location of the shadows produced by code wheel 24. Another method of discernment is to carefully control the size and location of light detectors 26 relative to the lines and windows of code wheel 24.
Code wheel 24 is mostly translucent with a series of opaque radial lines 32 near the periphery. Code wheel 24 is attached to rotating shaft 30. The stationary mask 28 has a matching set of opaque lines 34. Light from source 22 passes through the translucent portions of code wheel 24 and then through the translucent portions of mask 28, eventually falling upon detector 26. The amount of light falling upon detector 26 depends upon the alignment of the translucent portions of code wheel 24 to the translucent portions of mask 28. When the translucent portions of code wheel 24 align with opaque portions 34 of mask 28, light is blocked from detector 26. When the translucent portions of code wheel 24 align with the translucent portions of mask 28, light passes through to detector 26. The amount of light falling on detector 26 is a direct indication of the location of code wheel 24 relative to mask 28. The output of light detector 26 is a periodic function of the rotational angle of code wheel 24 and might look like the waveform shown in FIG. 2. It is to be understood that the term xe2x80x9crotational anglexe2x80x9d may also be referred to herein as xe2x80x9cangular displacementxe2x80x9d, xe2x80x9crotational positionxe2x80x9d, or xe2x80x9cangular positionxe2x80x9d.
Position changes can be coarsely tracked by counting the number of cycles traversed. Finer resolution can be obtained by observing the level of the output of detector 26 within each cycle. If the output of detector 26 were an ideal triangle wave, the entire fine resolution portion of the position measurement could be accomplished with one encoder channel. With current technology, this is not a realistic expectation. There is also some ambiguity with just one channel since two different positions within each cycle produce the same output magnitude. This ambiguity is overcome by a two channel encoder 36 (FIG. 3) including a second mask 29 and a second light detector 40 which are aligned with respect to the first mask 28 and light detector 26 so as to produce a signal that is 90 electrical degrees out of phase with the first signal shown in FIG. 4. The phase relationship of these two signals also helps determine direction of motion. Optionally, a second light source 38 in alignment with the second mask 29 and second light detector 40 may be included.
A two channel encoder is useful in tracking relative position changes of a rotating shaft. In the case where the absolute position of the shaft is required, an additional reference mark is needed. This is commonly accomplished by adding a third channel called the index channel with an associated light source 43 and index light detector 42 (FIG. 5). A single mark 44 detectable by optical sensor 42 is added to code wheel 24. A pulse occurs on the index channel once per revolution of code wheel 24 as index mark 44 passes sensor 42 thus indicating the absolute position of shaft 30.
Historically, most optical encoders have provided digital outputs. This is accomplished by xe2x80x9csquaring upxe2x80x9d the light detector outputs as shown in FIG. 6. Not xe2x80x9csquaring upxe2x80x9d the sensor signals, but instead processing them while they are still in their analog form can produce higher resolution.
In summary, given an optical encoder with two quadrature analog outputs and an index signal, absolute position is determined in the following manner. First, index mark 44 is found. The encoder is advanced until a pulse is seen on the index channel. Upon seeing this pulse, a count is started to keep track of the number of cycles of either channel A or channel B that have been traversed. In between discrete cycle counts, fine position resolution is achieved by examining the strength of the signals on both channels A and B.
What is needed in the art is a method and apparatus for converting the outputs from an analog position encoder into high precision, digital position data.
The present invention provides a high precision analog encoder system which uses the same basic optical sensor employed by other digital encoders but achieves hundreds of times the resolution.
The invention comprises, in one form thereof, a method of determining a feed position of a print medium in an imaging apparatus. A feed roll with a central feed shaft carries the print medium such that a rotational position of the feed shaft has a predetermined relationship with the feed position of the print medium. An encoder device, connected to the feed shaft, produces a first periodic signal and a second periodic signal approximately 90 degrees out of phase with the first. Each periodic signal is a function of the rotational position of the feed shaft. Each period of each signal corresponds to a predetermined rotational distance of the feed shaft. A modified first signal is created. The slope of the modified first signal has the same sign at each rotational position of the feed shaft. The magnitude of the slope of the modified first signal is equal to the magnitude of the slope of the unmodified first signal at each rotational position of the feed shaft. A modified second signal is created. The slope of the modified second signal has the same sign at each rotational position of the feed shaft. The magnitude of the slope of the modified second signal is equal to the magnitude of the slope of the unmodified second signal at each rotational position of the feed shaft. The modified first signal is added to the modified second signal to thereby create a summation signal. The summation signal has a plurality of local minimum values, a plurality of local maximum values, and a plurality of substantially linear segments. Each linear segment interconnects a corresponding local minimum value with an adjacent local maximum value. A periodic modified summation signal is created by adding a corresponding constant value to each linear segment to thereby create a plurality of shifted linear segments such that each shifted linear segment extends between a substantially equal minimum value and a substantially equal maximum value. Each shifted linear segment represents one cycle of the periodic modified summation signal. A number of completed cycles of the modified summation signal are counted. A value of the modified summation signal at a selected point in time is determined. The feed position of the print medium is calculated based upon the number of cycles counted and the determined value of the modified summation signal.
An advantage of the present invention is that the rotational position of a shaft can be precisely determined with a simple, robust system that is well adapted to high volume manufacturing.
Another advantage is that the system of the present invention can be used with an optical encoder whose output is non-sinusoidal.
Yet another advantage is that amplitude variations between channels, between encoders, and over time are automatically compensated for.
A further advantage is that the present invention needs neither a division operation to compute a tangent, nor a means of computing an arctangent, such as a look-up table, as are required by known optical encoder systems.